This invention relates to frequency modulation measurements and more particularly to a novel method and apparatus for accurately testing and adjusting the modulation sensitivity and, hence, the frequency deviation of a frequency modulated oscillator (FMO). A technique for measuring the frequency deviation of a frequency modulated oscillator is disclosed in U.S. Pat. No. 3,549,997, issued Dec. 22, 1970, inventor D. Rotzel, wherein a frequency modulated signal and a reference signal are separately used to derive zero crossing pulses which are separately coupled to counters via gates. The frequency modulated signal is additionally coupled to a pulse shaper via a discriminator which generates a pulse that opens the gates to initiate counting. Each gate is separately closed by its associated counter when a predetermined count is reached for that counter. The time elapsing between the closing of the respective gates is a measure of the frequency deviation. The time is measured by coupling the gate closing signals via an exclusive-OR gate to another gate which allows pulses from a timing pulse generator to reach a direct reading counter. One problem with this technique is that the time measurement will vary depending upon whether the frequency modulated signal is started during its positive half-cycle when its frequency will be greater than that of the reference frequency, or during its negative half-cycle, when its frequency will be less than the reference frequency. In order to reduce the effects of short time fluctuations, the deviation must be determined by taking an average value of the measurements. Another frequency deviation measuring system is disclosed in U.S. Pat. No. 4,122,391, issued Oct. 24, 1978, inventors Maurice C. Harp, et al., wherein a calibrated unipolar signal is used to deviate a frequency modulated oscillator by a predetermined amount f.sub.s, from a carrier f.sub.c. An automatic frequency control circuit is normally connected in the feedback control path and, when connected, would operate to bring the carrier back on frequency and a meter connected in the control circuit would read this departure from normal. However, during the test for the correct deviation, a frequency shifter is connected between the output of the frequency modulated oscillator and the input of the automatic frequency control circuit. The frequency shifter is set to provide the frequency shift necessary to set its output to that of the carrier alone if the test signal has been deviated the correct amount. In this case, the indication on the test meter would be the same as the unmodulated carrier, indicating correct adjustment. However, if the deviation caused by the calibrated unipolar signal is not equal to that incorporated in the frequency shifter, the meter reading would reflect this fact and adjustment would be necessary. In one embodiment of the invention, a programmable counter is used in the automatic frequency control circuit and the program is modified a fixed amount when the test function switch is set to apply the unipolar pulse to the frequency modulated oscillator. Thus, the programmable counter replaces the frequency shifter in the deviation test circuit. The disadvantages of this technique lie in the use of a unipolar pulse rather than a symmetrical square wave for the modulation test signal. The use of the square wave allows the reduction in time for adjusting the deviation and a relative decrease in noise of the detected signal, hence a usable increase in sensitivity over the unipolar pulse approach.